The present disclosure relates to a fuel cell, and particularly to a fuel cell in which countermeasure is taken against heat generated in a power generation cell, the fuel cell being suitable for miniaturization.
In recent years, in portable electronic apparatuses such as a cellular phone, a notebook-size personal computer, a digital camera, a camcorder, and the like, their functions have been advanced and diversified, and accordingly, power consumption tends to increase. Therefore, loads on power supplies have increased.
As power supplies for these portable electronic apparatuses, small primary cells or secondary cells are generally used. One of the characteristics of cells is an energy density. The term “energy density” represents a quantity of energy which can be taken out per unit mass or unit volume of a cell. Cells used for portable electronic apparatuses are required to be improved in energy density in order to comply with higher function and multi-function of electronic apparatuses.
When energy possessed by a primary cell is discharged, the cell cannot be reused. Although a primary cell has convenience that a portable electronic apparatus can be operated again by exchanging the cell with another cell, primary cells have a low energy density, many cells are required to be carried for driving a portable electronic apparatus which consumes much electric power, and thus primary cells are unsuitable as power supplies of portable electronic apparatuses.
Use of secondary cells has the advantage that even if energy stored in the cells is discharged, the cells are reproduced by charging and can be reused. However, the energy density is not sufficient to drive a portable electronic apparatus with large power consumption for a long time, and a charger and a power supply are required for charging, thereby limiting operation environments. Also, there is the problem that charging requires much time.
As described above, it is difficult to comply with drive of various portable electronic apparatuses for a long time by conventional primary cells, secondary cells, or extensions thereof, and power supplies suitable for drive for a longer time and based on a different principle are expected. One of such power supplies is a fuel cell. A fuel cell includes an anode, a cathode, and an electrolyte, wherein fuel is supplied to the anode side, and an oxidizer is supplied to the cathode side. At this time, an oxidation-reduction reaction takes place to oxidize the fuel with the oxidizer, and chemical energy possessed by the fuel is efficiently converted to electric energy.
Since a fuel cell is a power generator which generates electric power using a chemical reaction between fuel and an oxidizer, the fuel cell can be continuously used as a power supply by continuously using air oxygen as the oxidizer and supplying the fuel from the outside unless the fuel cell is damaged. Therefore, if fuel cells can be miniaturized, they become high-energy-density power supplies suitable for portable electronic apparatuses and not requiring charging.
Various types of fuel cells have already been proposed or made on an experimental basis, and some of them have been put into practical application. The properties of fuel cells significantly vary depending on the electrolytes used, and the fuel cells are classified into various types on the basis of the electrolytes used. Among these, polymer electrolyte fuel cells (PEFC) using proton conductive polymer membranes as electrolytes are operated at a low temperature of about 30° C. to 130° C. without using electrolytic solutions, and thus they can be miniaturized and are optimum as power supplies for portable electronic apparatuses.
As fuel for fuel cells, various combustible materials such as hydrogen, methanol, and the like can be used. However, gaseous fuel such as hydrogen has a low density and is not suitable for miniaturization because a high-pressure storage cylinder or the like is required for increasing the density. On the other hand, liquid fuel such as methanol has a high density compared with gases and can be easily stored, and thus liquid fuel is overwhelmingly advantageous as fuel for fuel cells for small apparatuses. Therefore, if fuel cells using liquid fuel can be miniaturized, inconventional power supplies for portable electronic apparatuses, which can be driven for a long time, can be realized.
In particular, direct methanol fuel cells (DMFC) in which a reaction is effected by supplying methanol directly to an anode of PEFC require no reformer for taking out hydrogen from fuel, are simply configured, and are easily miniaturized. The energy density of methanol is theoretically significantly higher than that of general lithium ion secondary cells. As described above, DMFC is considered most suitable as a power supply for portable electronic apparatuses which are increasingly miniaturized and made multifunctional and higher functional.
In DMFC, methanol as fuel is oxidized to carbon dioxide in an anode catalyst layer as shown by the following expression (1):Anode: CH3OH+H2O→CO2+6H++6e−  (1)The hydrogen ions produced in this reaction move to the cathode side through a proton conductive polymer electrolyte membrane held between the anode and cathode and react with oxygen in a cathode catalyst layer as shown by the following expression (2):Cathode: 6H++(3/2)O2+6e−→3H2O  (2)The reaction taking place over the whole of DMFC is represented by combination of expressions (1) and (2), i.e., the following expression (3):Whole DMFC: CH3OH+(3/2)O2→CO2+2H2O  (3)
DMFC is roughly divided into a liquid supply type and a gas supply type according to a method for supplying methanol to an anode. The liquid supply type is a method of supplying liquid fuel as it is, in which an aqueous methanol solution is supplied to the anode using a pump. In DMFC, water is consumed by the electrode reaction (1) on the anode. Therefore, in many DMFC, an aqueous methanol solution is supplied to the anode to compensate for a loss of water. However, this type causes methanol crossover in which methanol passes through the polymer electrolyte membrane from the anode side to the cathode side and easily causes the problem of decreasing the efficiency of utilization of methanol.
The gas supply type is a method of supplying vaporized methanol to the anode, in which the liquid fuel stored in a fuel tank is sent to a vaporizing chamber with a pump and naturally evaporated in the vaporizing chamber or forcedly evaporated by heater heating (refer to Patent Publication No. 3413111). There is a method of naturally evaporating the fuel in a fuel tank or forcedly evaporating the fuel by heater heating in the fuel tank.
In the gas supply type, the water produced on the cathode is backward diffused to the anode side to prevent residence of water on the cathode, compensate for the water consumed by the electrode reaction (1) on the anode, maintain water in the polymer electrolyte membrane by self humidification, and allow the polymer electrolyte membrane to exhibit high proton conductivity. The gas supply type is known as a method which causes relatively little methanol crossover. In addition, swelling of the polymer electrolyte membrane can be suppressed, thereby stabilizing a membrane-electrode assembly.
In both the liquid supply type and the gas supply type, air is supplied to a power generation portion by a forced method using a pump or fan or a method using natural diffusion or convection of air without using a pump or fan.
As a method of stabilizing the supply of methanol and air, a method of controlling the supply rate using a pump, a blower, or a heater can be used. However, such an auxiliary part inhibits miniaturization of DMFC and has the side of impairing a characteristic of DMFC, i.e., the high energy density. Therefore, a power supply for portable electronic apparatuses preferably uses, as the methanol supplying method, the gas supply type of naturally evaporating fuel in a fuel tank and uses the method utilizing natural diffusion or convection as the air supplying method.
However, in this case, the fuel supply rate is strongly affected by the temperature of a space in which methanol is evaporated, and thus when the temperature of the space is excessively increased by heat generated with power generation, the fuel is excessively supplied, causing methanol crossover. In addition, the gas supply type requires that the water produced on the cathode side inversely diffuses in the electrolyte membrane and is supplied to the cathode side, but when the temperature of the polymer electrolyte membrane becomes excessively high, the water produced on the cathode side and the water contained in the electrolyte membrane are lost by evaporation, thereby causing the problem of making it impossible to supply water necessary for a reaction on the anode.
Overheating with the heat generated with power generation has the property that the situation gradually worsens once it occurs. For example, when the fuel is excessively supplied due to overheating of the space for evaporating methanol, thereby causing methanol crossover, the situation is easily caught in a vicious circle in which the crossover methanol is oxidized on the cathode, and the generated heat further increases the temperature of the methanol evaporation space, thereby further causing excessive supply of the fuel and methanol crossover. In addition, when water is lost from the polymer electrolyte membrane at a high temperature, there occurs a vicious circle where the internal resistance of the polymer electrolyte membrane is increased, and consequently, resistance heat generation is increased, thereby further increasing the temperature of the polymer electrolyte membrane due to the generated heat.
As a method of stabilizing the temperature by controlling the heat generated by power generation without using a pump or a fan, there is a method of providing radiation means including a radiation fin at a position in contact with outside air. An example of a fuel cell provided with a radiation fin is proposed in, for example, Japanese Unexamined Patent Application Publication No. 2005-108717 (pages 3, 5, 6, and 9, particularly paragraphs [0005] and [0041], FIG. 7).
FIG. 4 is a sectional view showing a method of cooling an electromotive portion of the fuel cell disclosed in Japanese Unexamined Patent Application Publication No. 2005-108717. The electromotive portion 100 is provided with a stack of three power generation cells 101a to 101c, a fuel supply passage 105 and a fuel discharge passage 106 are provided for supplying or discharging fuel to or from the power generation cells, and an air supply passage 107 and an air discharge passage 108 are provided for supplying or discharging air to or from the power generation cells. In addition, radiation fins 104 are provided on side walls 103a and 103b which constitute the side surfaces extending along the stacking direction of the power generation cells 101a to 101c. On the other hand, radiation fins are not provided on the end plates 102a and 102b which constitutes the end surfaces perpendicular to the stacking direction of the power generation cells.
In Japanese Unexamined Patent Application Publication No. 2005-108717, the characteristic of the cooling method for the electromotive portion 100 is described as follows: The radiation fins 104 provided on the side walls 103a and 103b radiate heat of the power generation cells 101a to 101c to the surroundings and cool these cells. Consequently, overheating of the cells is prevented. The side walls 103a and 103b extend along the stacking direction of the power generation cells 101a to 101c and position to face the plurality of power generation cells. Therefore, the plurality of power generation cells 101a to 101c can be uniformly cooled, and the occurrence of a difference in temperature between the power generation cells can be prevented. Further, the fuel discharge passage 106 and the air discharge passage 108 on the discharge side at the highest temperature extend in the stacking direction of the cells and are thus efficiently cooled with the radiation fins 104 provided on the side surfaces of the electromotive portion 100.
As a result, a difference in temperature and output variation between the plurality of power generation cells 101a to 101c can be suppressed, thereby permitting stable power generation. At the same time, breakage such as polarity inversion or the like in the power generation cells is prevented, thereby providing a fuel cell with improved reliability.
In the electromotive portion 100 disclosed in Japanese Unexamined Patent Application Publication No. 2005-108717, radiation fins are not provided on the end plates 102a and 102b which constitutes the end surfaces perpendicular to the stacking direction of the power generation cells 101a to 101c. In Japanese Unexamined Patent Application Publication No. 2005-108717, as the reason for this, it is described that when radiations fins are provided, for cooling, on the end plates 102a and 102b which constitutes the end surfaces in the electromotive portion 100 configured by stacking the plurality of power generation cells 101a to 101c, a temperature difference easily occurs between the power generation cells 101a and 101c provided at the ends in the stacking direction and the power generation cell 101b provided at the center, and consequently, output varies between the power generation cells and is not stabilized, leading to breakage such as polarity inversion due to a temperature difference in some cases.
That is, the cooling method for the electromotive portion 100 shown in FIG. 4 is a cooling method employed for minimizing variation between the stacked power generation cells as the high-priority issue and is a second-best method which is inevitably used because three or more power generation cells are stacked. Therefore, the structure of the electromotive portion 100 is not the best structure when consideration is given to cooling of one power generation cell with radiation fins with highest efficiency.
It is therefore desired to provide a fuel cell capable of preventing destabilization of power generation due to heat generated in an electrochemical device portion and preventing decrease in generation efficiency.